Use FlatStringReader in JSON stringifier.
Use FlatStringReader in JSON stringifier.

R=ishell@chromium.org

Review URL: https://codereview.chromium.org/736543003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#25391}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh